A late surviving Pliocene seal from high latitudes of the North Atlantic realm: the latest monachine seal on the southern margin of the North Sea

Abstract

BackgroundThe family of true seals, the Phocidae, is subdivided into two subfamilies: the southern Monachinae, and the northern Phocinae, following the subfamilies’ current distribution: extant Monachinae are largely restricted to the (sub-)Antarctic and the eastern Pacific, with historical distributions of the monk seals of the genus Monachus in the Caribbean, the Mediterranean and around Hawaii; and Phocinae to the northern temperate and Arctic zones. However, the fossil record shows that Monachinae were common in the North Atlantic realm during the late Miocene and early Pliocene. Until now, only one late Pliocene record is known from the Mediterranean, Pliophoca etrusca from Tuscany, Italy, but none from farther north in the North Atlantic.MethodsWe present the description of one partial phocid humerus collected in the early 20th century from the Antwerp area (Belgium), with an assessment of its stratigraphic origin using data from the literature.ResultsThe studied humerus was recovered during construction works at the former Lefèvre dock in the Antwerp harbour (currently part of the America dock). Combining the information associated to the specimen with data from the literature and from local boreholes, the upper Pliocene Lillo Formation is ascertained as the lithological unit from which the specimen originates. Morphologically, among other features the shape of the deltopectoral crest and the poor development of the supinator crest indicates a monachine attribution for this specimen. The development of the deltopectoral crest is closer to the condition in extant Monachinae than in extinct Monachinae.DiscussionThe presented specimen most likely represents a monachine seal and a literature study clearly shows that it came from the latest early to late Pliocene Lillo Formation. This would be the first known monachine specimen from the latest early to late Pliocene of the North Sea, and more broadly from the northern part of the North Atlantic realm. This humerus differs from the humerus of P. etrusca and suggests a higher diversity of Monachinae in the latest early to late Pliocene than previously assumed.

True seals achieved global distribution by breaking Bergmann's rule

“…Likewise, the tropical affinities of monk seals appear to be a derived condition. Overall, the modern contrast between polar and tropical phocids appears relictual, and largely reflects local extinctions of phocids at mid-latitudes during the late Neogene (Avery and Klein 2011;Valenzuela-Toro et al 2013;Pimiento et al 2017;Dewaele et al 2018;Rule et al 2019). Trait evolution is best assessed based on phylogenies including both extant and extinct taxa (Quental and Marshall 2010).…”

“…5, 6; Tables A8, A9). Body size evolution in true seals was thus not obviously driven by temperature, and instead may reflect nutrient availability and/or feeding ecology (Dewaele et al 2017(Dewaele et al , 2018Rule et al 2021a).…”

A new balaenopterid species from the Southern North Sea Basin informs about phylogeny and taxonomy of Burtinopsis and Protororqualus (Cetacea, Mysticeti, Balaenopteridae)

“…Vandenberghe et al (1998) estimated the age of the Luchtbal Sand Member as late Zanclean, about 3.5 Ma, based on benthic foraminifer zone B11, benthic mollusk association BM22A and otholith zone 18 (Vinken, 1988). De Schepper, Head & Louwye (2009) estimated the maximum age of the Luchtbal Sand Member at 3.71 Ma and the minimum at 2.72-2.74 Ma (De Schepper, Head & Louwye, 2009), confining the deposit to the latest Zanclean-earliest Piacenzian (Dewaele, Lambert & Louwye, 2018).…”